The present invention relates to a valve operating apparatus for an internal combustion engine and more particularly, but not exclusively, to a valve operating apparatus for an internal combustion engine equipped with an overhead camshaft.
As is well known in the art, camshafts have cam lobes integrally formed as a part thereof for opening and closing intake and exhaust valves, each cam lobe comprising a cam nose section by which a topmost point of valve travel is defined; opening and closing flank sections for lifting a member, for example a tappet, a rocker arm or the like, of a valve train so as to open and close the valve; and ramp sections which connect the opening and closing flank sections to a base section of the cam having a substantially semi-circular periphery coaxial with the camshaft. A ramp height of the cam lobe, which is defined by the difference between the radial distance of a juncture of the ramp and flank sections from the center of the camshaft and the radius of a base circle of the base section, is provided for the reason that a smaller clearance is needed between the base section of the cam and the member of the valve train associated with the cam not only because if the member of the valve train always rides on the base section of the cam, the valve is possibly disabled to close from the result of a thermal expansion of the valve train during engine operation but also because that the cam is needed to lift the valve excessively in order to cancel a deflection of the valve train, in particular a valve stem, which is caused before the valve actually opens after the member of the valve train has been brought into contact with the flank section of the cam lobe. Furthermore, if there is not provided a ramp section, it will be hard to connect the flank section to the base with continuity or without uneveness; objectionable noises and vibrations will occur when the member of the valve train rides on or over the flank section of the cam lobe.
Therefore, for a smooth and quiet operation of the engine, a proper lamp height has to be provided in every cam lobe. It the cam has a cam lobe with an improper ramp height, it develops a serious problem such as a bouncing. Specifically, when the valve moves down from its topmost to its lowermost point of travel, in particular immediately before a valve head of the valve is closed tightly against its valve seat, the valve reduces rapidly its speed of movement, resulting in a resilient deflection of the valve train, in particular of the valve stem thereof. As long as the cam lobe of the cam has a ramp height properly designed, even though a practical valve lift becomes smaller than a designed valve lift due to the resilient deflection of the valve train, the cam substantially cancels the deflection. As a result, the valve head can be seated in the valve seat more gently and quietly.
On the other hand, if the cam has a cam lobe with an improper or smaller ramp height, it is impossible to cancel the resilient deflection of the valve train. This develops a problem that the valve head impulsively clashes against the valve seat and, therefore, bounds to re-open immediately after closing. This re-opening of valve is well known in the art as a bouncing. If in fact such a bouncing occurs on the valve, the valve train, in particular the valve stem, receives an undesirable, excessively strong impulsive force and, thereby, is sometimes damaged or confuses an intake and exhaust operation of the engine. Because bouncing occurs depending on the impulsive force which increases as an increase of engine r.p.m., the impulsive force is one of the factors which govern a practical maximum engine r.p.m.
Meanwhile, in overhead valve engines, for example inline four cylinder overhead camshaft engines of the type having a camshaft with a plurality of operating cams, four every cylinder, the camshaft is mounted for rotation on the engine by a plurality of journal bearings. The camshaft, in particular for intake valves, is provided at one end with a timing sprocket or timing gear driven by a toothed drive belt and is coupled at the opposite end to a distributor. One such camshaft is disclosed in Japanese Patent Unexamined Publication entitled "Valve Operating Apparatus Used In A Multi-Cylinder Internal Combustion Engines", laid open Mar. 25, 1983.
The intake camshaft thus constructed has outermost cams which are oppositely located outside the outermost journal bearings, respectively. In other words, each outermost cam is supported in the form of a cantilever by and overhangs the outermost journal bearing. Due to this cantilever supported outermost cam, the opposite end portions of the camshaft extending outside the outermost journal bearings tend to become weaker in rigidity more than the rest portion of the intake camshaft. In more detail, the end portion with timing gear attached thereto is not so weak in comparison with the other opposite end portion with the distributor coupled thereto because it will receive a force from the tensioned drive belt. For this construction of the camshaft, the end portion of the intake camshaft coupled to the distributor is apt resiliently to deflect.
Owing to such a deflection of the member of the valve train and the intake camshaft itself, the outermost valve in association with the outermost cam generally becomes short in travel or valve lift more than its intended or designed valve lift. This means that the cam lobe of the outermost cam actually has an improper, namely smaller, ramp height and makes the outermost valve cause a bouncing due to an excessive impulsive force exerted on the valve seat. Therefore, the increased impulsive force against the valve seat induces a bouncing of valve at a lower engine r.p.m., thereby reducing the practical maximum engine r.p.m. of the engine. Usually, in overhead valve engines, the practical maximum engine r.p.m. thereof is governed by a critical implusive force inducing an undesired bouncing on the valve operated by the outermost cam which is produced at a engine r.p.m. lower or less than engine speeds at which an undesired bouncing is induced on the valves operating the remaining cams.
As apparent from the above description, the conventional valve operating apparatus having a camshaft which has a plurality of cams formed with a same cam lobe of which the outermost cam is located outside the outermost journal bearing limit practical engine speed and is harder to operate with a high efficiency.